Hydrogen and hydrocarbon conversion reactions including such as steam reforming, water-gas shift reactions, methanol synthesis and catalytic combustion are well known. These reactions are usually carried out at temperatures between 150 and 1000° C. Currently these reactions are industrially run using catalyst pellets which consist of an active catalytic metal or metal oxide deposited on high surface area ceramic pellets.
Foam or monolith catalysts are known that have three layers (1) porous support, (2) interfacial layer, and (3) catalyst metal as described in [1]. In making these catalysts, the interfacial layer has been deposited by various methods including solution impregnation techniques. The catalyst layer may be deposited by solution impregnation techniques. The interfacial layer has greater surface area than the porous support whereas the porous support has greater mechanical strength than the interfacial layer.
The porous support may be a metal or ceramic foam. Metal foams are highly thermally conductive and easy to machine. The sponge-like mechanical properties allow convenient sealing in a reaction chamber via mechanical contact. The closely matched thermal expansion between the metal foam and the housing reaction chamber minimizes cracking of the porous support and minimizes gas channeling around the porous support at higher reaction temperatures. Pestryakov et al prepared metal foam supported transition metal oxide catalysts with [1] and without [2] an intermediate gamma-alumina layer for the oxidation of n-butane. Kosak [3] examined several approaches to disperse precious metals on various metal foams where the surface was pre-etched with HCl solution, and reported that electroless deposition provides the best adhesion of precious metals to the foam supports. Podyacheva et al. [4] also synthesized foam metal supported LaCoO3 perovskite catalyst with a porous alumina intermediate for methane oxidation. Despite all of the potential advantages with metal foam supported catalysts, metal foam has low corrosion resistance and its nonporous and smooth web surfaces have provided poor adhesion to ceramic materials, and these materials are prone to spalling of interfacial layers after thermal cycling because of the mismatch in thermal expansion.
In order to increase corrosion resistance, methods such as diffusion alloying with Al, Cr, and Si have been used to fabricate ferritic steels, which are typically used for the manufacturing of high temperature furnace elements (about 1200° C.) [5]. When the aluminum containing ferritic steels are appropriately heat-treated, aluminum migrates to the alloy surface and forms a strongly adhering oxide film which is resistant to oxygen diffusion. Such ferritic steel foils have been used to fabricate metal monoliths with >10 ppi (pores per inch) open cells [6]. However, the search for the similar alloy foams with pores suitable for catalytic applications (<20 ppi, 80 ppi preferred) has been fruitless. This has been attributed to both the immature methods for making the finer Al-ferritic steel foams and the lack of the alloy precursors for making the foams.
Hence, there is a need in the art of supported catalysts for a porous support of a foam that is resistant to corrosion or oxidation and resists cracking of the interfacial layer.